“From late October to late December
2019, a negative SAM contributed to the lack of rainfall over parts of eastern
Australia and the higher than average temperatures in the south and east”
said Jonathan Pollock, Climatologist at the Bureau of Meteorology.

The Southern Annular Mode, or SAM as it is
often called, refers to the non-seasonal northward or southward shift of the
belt of westerly winds over the Southern Ocean. This belt lies on the southern
side of the subtropical ridge — a band of high pressure that moves north and
south with the seasons and is associated with drier and more stable conditions.

For most of the colder half of the year
(May to October) the subtropical ridge sits over Australia and during the
warmer half (November to April) it is generally located to the south of the mainland.
The changing position of the subtropical ridge is the reason why cold fronts
and troughs can penetrate well into southern Australia during winter, but
during summer they are far less common.

SAM in summer

SAM varies between three phases: positive,
neutral and negative. A neutral phase of SAM means the belt of westerly winds
is located near its average position for that time of year.

But when SAM is in a positive or negative
phase the position of these westerly winds moves south or north (Figure 2).

A positive SAM in summer means the
westerlies are even further south than usual, allowing tropical onshore
easterlies to bring more warm moist air into eastern Australia, often resulting
in above average rainfall over parts of New South Wales, Victoria, Tasmania and
Queensland.

But a negative SAM in summer means less
moist onshore flow from the east and typically less rainfall than usual over
eastern Australia. Instead, dry continental air from inland Australia is pushed
into this region, bring warmer and drier conditions and often increased fire
risk.

Figure 2. Summer impacts of the Southern Annular Mode (SAM). During a positive phase of the Southern Annular Mode (SAM), westerlies shift further south allowing moist onshore flow across much of eastern Australia. In a negative phase, eastern Australia tends to see more dry continental airflow. Source: Bureau of Meteorology.

SAM in winter

SAM’s impact can be quite different in
winter because of the position of the subtropical ridge. The westerly winds are
further north at that time of year and have more influence on our climate
(Figure 3).

When SAM is in a positive phase during
winter the westerly winds (and cold fronts) move further south than usual,
meaning that places like southwest Western Australia, western Tasmania and
parts of South Australia, Victoria and southern New South Wales typically miss
out on some of their usual winter rainfall.

Conversely, when SAM is in a negative phase
during winter, southern Australia can receive more cold fronts and storm
systems, and typically more rainfall.

Figure 3. Winter impacts of the Southern Annular Mode (SAM). In its positive phase westerly winds shift further south than normal bringing wetter conditions to eastern Australia and drier to the south. In its negative phase the winds shift further north than usual, and these affects are reversed. Source: Bureau of Meteorology.

What else do we know about SAM?

The rainfall impacts from SAM are more
pronounced during winter and summer compared with autumn and spring. In autumn,
Australia’s climate typically sees very little effect from SAM, while in
spring, the effect on rainfall resembles a weak summer pattern. But autumn and
spring tend to be the times of the year when SAM has greatest influence on
extreme heat.

“In the second half of spring and
early summer 2019, SAM spent more time than usual in its negative phase. The
belt of westerlies was further north than usual. That meant reduced rainfall in
parts of the east because of decreased onshore flow” said Mr. Pollock.
“At the same time, some southern parts of Victoria, or west-facing
southern parts, recorded near average rainfall, and parts of western Tasmania above
average rainfall despite the drying influence of one of the strongest positive Indian
Ocean Dipole events on record.”

Negative SAM was one factor influencing the
fire weather conditions in some parts of Australia towards the end of 2019. When
the mode is in a prolonged negative phase, fire weather conditions in Australia
are typically worse, particularly in New South Wales. A negative SAM is often
associated with an early start to the bushfire season as it was in 2013 and
2018. It isn’t the only culprit though with climate drivers El Niño and the positive
Indian Ocean Dipole also playing a role.

Most positive or negative SAM events last
for around ten days to two weeks, though longer periods may also occur. Late
2019’s persistent negative SAM lasted around two months, only dipping back into
the neutral range for a few days in November.

April to October rainfall across
southeastern and southwestern Australia has declined in recent decades. The
drying trend is linked with a trend towards more positive SAM (Figure 4); a
known response to global warming.

Like our other climate drivers, SAM can
influence weather patterns over parts of Australia. But unlike El Niño–Southern
Oscillation and Indian Ocean Dipole events which persist for months at a time, SAM
events generally only last for around ten days to two weeks. The Bureau of Meteorology’s
Climate
Outlooks provide guidance on rainfall and temperature patterns for the
weeks, months and seasons ahead. The influence of SAM (and our other climate
drivers) is explained in the Climate Outlooks summary information.